Apoptosis plays a crucial role in both the development and aging of the central nervous system (CNS). Aberrant neuronal apoptosis has also been linked to the pathogenesis of several neurodegenerative diseases and in the loss of neurons following stroke or trauma. The model system we study, the olfactory neuroepithelium (OE), contains a simple, accessible group of Olfactory Receptor Neurons (ORNs), each of which undergo apoptosis every 4-6 weeks throughout adulthood. Because the OE has the unique ability to regenerate its entire neuronal population repeatedly and re-establish functional connectivity, olfactory function is thus retained. Because of its accessibility for experimental manipulation in vitro and in vivo, the OE is an amenable model system for examining molecular and cellular mechanisms that drive the separate, but related, processes of neurogenesis and neuronal apoptosis. The overall goal of this project is to use the olfactory system to dissect the molecular pathways by which mammalian neurons undergo apoptotic death, focussing on the role s of specific members of the caspase family of proteases. The overall hypotheses are that the pathways of neuronal apoptotic death are composed of multiple, causally-linked steps mediated by different caspases, and that the spatiotemporal nature of these steps depends on the initiating causes of death. The primary focus is to study synchronous ORN apoptosis induced by surgical removal of the ORN target, the olfactory bulb in comparison with inducing ORN apoptosis by trophic factor withdrawal (excitotoxic removal of target neurons with NMDA) and over-stimulation. We have already demonstrated that caspase 3 and 9 play central roles in ORN apoptosis and that they can initiate apoptotic pathways at the axon terminal. To define their role further, in Aim One we shall examine pathways leading to caspase -3 and -9 activation in mature ORN apoptosis in vivo and in Aim Two, we will re-establish and test these pathways during ORN apoptosis in vitro. In Aim Three we examine the role of caspases in developmental ORN apoptosis, focusing on caspase 3 and 9 null mutant mice. In Aim Four we will test inhibitor and adenoviral expression approaches to prevent or delay ORN apoptosis in vitro and in Aim 5, we will generate an OMP-Cre mouse for the purpose of applying a conditional knockout strategy to examine apoptotic pathway regulation only in the mature ORN population.